1. Technical Field
The present invention relates to fluorescent X-ray analysis. More specifically, the present invention relates to a concentration measuring method and a fluorescent X-ray spectrometer for measuring the concentration of a measurement object, such as sulfur, included in a sample, e.g. an alcohol fuel such as methanol or ethanol, using fluorescent X-ray analysis.
2. Description of Related Art
Fluorescent X-ray analysis is an analytical method for irradiating a sample with primary X-rays, detecting fluorescent X-rays generated from the sample and making a qualitative analysis or a quantitative analysis of elements contained in the sample on the basis of a spectrum of the fluorescent X-rays. A fluorescent X-ray spectrometer for making a fluorescent X-ray analysis is composed of; an X-ray tube for generating primary X-rays; an X-ray detector constituted of a semiconductor detector, a proportional counter or the like; an analyzer for analyzing the wavelength distribution or the energy distribution of X-rays detected by the X-ray detector; and the like. For making a fluorescent X-ray analysis, a sample is irradiated with primary X-rays generated by the X-ray tube, fluorescent X-rays generated from the sample irradiated with the primary X-rays are detected by the X-ray detector, and a spectrum of the detected fluorescent X-rays is analyzed by the analyzer.
Such fluorescent X-ray analysis can be used for measuring the concentration of impurities included in a liquid fuel. For example, the concentration of sulfur in a liquid fuel is measured using fluorescent X-ray analysis with the aim of reduction in sulfur, which is a hazardous component, included in a liquid fuel such as a diesel fuel. When a fluorescent X-ray analysis is made for a sample of a liquid fuel including sulfur, an X-ray spectrum including a signal of fluorescent X-rays of sulfur and a signal of scattered X-rays, which are primary X-rays scattered in the sample, is obtained. When the fluorescent X-ray intensity of sulfur to be obtained from the spectrum is denoted by S and the scattered X-ray intensity is denoted by B, a value (S/B) to be obtained by dividing S by B becomes a function approximately proportional to the concentration of sulfur in the sample. A calibration curve representing the relation between (S/B) and the concentration is preset using a standard sample having a known sulfur concentration. The sulfur concentration can be obtained by comparing the value of (SIB), which is obtained by a fluorescent X-ray analysis of a sample having an unknown sulfur concentration, with the calibration curve.
In recent years, standards for the regulation of hazardous components included in a liquid fuel such as gas oil has been getting strict and it is required to measure a low sulfur concentration. In such a case, it is required to obtain the sulfur concentration from weaker fluorescent X-rays. A spectrum to be obtained from a fluorescent X-ray analysis includes a peak, which is referred to as a system peak. A system peak is a peak attributed to: X-rays scattered in the air in a fluorescent X-ray spectrometer; X-rays reflected at components in the fluorescent X-ray spectrometer; fluorescent X-rays generated by irradiation of components in a fluorescent X-ray spectrometer with X-rays; or the like, and is dependent on a fluorescent X-ray spectrometer. When a low sulfur concentration is to be measured, a signal of fluorescent X-rays of sulfur is small. Therefore, the effect of the system peak superimposed on a signal of fluorescent X-rays of sulfur becomes large and the value of (S/B) becomes unproportional to the concentration. In view of the facts, Japanese Patent Application Laid-Open No. 2001-91481 discloses a technique for measuring a low sulfur concentration in the presence of a system peak, by preliminarily obtaining a calibration curve designed in consideration of the effect of the system peak and measuring the sulfur concentration using the obtained calibration curve.